Nervous System

Myelin sheath in H and E slides

Spinal cord

Myenteric ganglion

Pacinean Corpuscle

Nervous Tissue:

The organization of cells in nervous tissue

The organization of organelles in neurons.

Nervous System Organisation:

-Central nervous system (CNS)-Brain & spinal cord

-Peripheral nervous system (PNS)-Outside CNS-Cranial & spinal nerves and associated ganglia

- PNS > sensory> motor - somatic: impulses transmitted

directly to skeletal muscle via one neuron - autonomic: impulse one neuron autonomic ganglion SM, CM, glands

Cells of the nervous system:

Glia

Neurons- perikaryon (soma, cell body)- dendrites- axon- axon terminals

Neurons:

- many shapes (unipolar, bipolar, pseudounipolar,multipolar)

Usually polarized

- cell body

- dendrites

- axon

- axon terminal

Dendrites:

Receive and integrate synaptic signals.**Dendritic spines**

Some neurons have many dendrites.

Some dendrites have many branches.

Ultrastructure similar to cell body.

Axon: one process, some have collateral branches.

-can be long in the PNS

-NO ribosomes

-Yes, neurofilaments, microtubules, mitochondria> believed to serve in regulation of diameter

Axon ensheathing cells are glia.

o Oligodendrocytes in the CNS.o Schwann cells in the PNS.

Ensheathing cells responsible for maintaining myelin sheath

In the CNS,

white matter contains myelinated axons

gray matter, no myelination

Other types of glia (neuroglia):

Astrocytes:

protoplasmic - gray matterfibrous - white matter- regulate ions and other molecules around neurons- release glucose from glycogen- have processes around blood vessels- form scar tissue

astrocyte

Oligodendrocytes: smaller than astocytes-some produce myelin

Microglia: small phagocytic cells-originate from bone marrow

Ependymal cells: - cuboidal or low columnar simple epith.- line the ventricles

Astrocytes – green

Microglia – red

Morphological reactions of injured axons in the PNS: PAGE 216

Chromatolysis: may last several months. •Nissl bodies disperse, peripheral nucleus.•Soma producing: Free ribosomes, protein, RNA and other molecules.•The axon and myelin sheath distal to the lesion degenerates as far as the axon collateral•sprouting of the axon into endoneurium•guiding by proliferating Schwann cells toward the target •regeneration in the presence of macrophages, fibroblasts, Schwann cells and basal lamina. These express growth factors, cytokines, up-regulate expression signal receptors.

Schwann cells form a sheath that guides the sprout to the targetSprout makes synaptic contact with targetOther sprouts degenerate

If no synaptic contact is made, the target cell degenerates.

Need connective tissue sheaths for the regeneration.These are absent in the CNS generally, no regeneration

In the CNS, microglia phagocytose the injured cellsGlial scar occupies the space.

Chromatolysis cont’d

Location of synapses:

Axodendritic synapses

Axosomatic synapses

Axoaxonic synapses

Dendrodendritic synapses

Ultrastructure of neurons:

Cell body - nucleus- rER and polyribosomes (Nissl bodies)- Golgi complex- mitochondria- lipofuscin granules (considered ‘aging’ pigment)- cytoskeleton: neurofilaments, microtubules

Axon hillock - no rER

Pacinian CorpusclePacinian Corpuscle

Nerve ending